Rice is one of the most important food crops in the world, particularly in Asia. Rice is a cereal grain produced by plants in the genus Oryza. The two most frequently cultivated species are Oryza sativa and Oryza glaberrima, with O. sativa being the most frequently cultivated domestic rice. In addition to the two domestic species, the genus Oryza contains more than 20 wild species. One of these wild species, Oryza rufipogon (“red rice” also referred to as Oryza sativa subsp. rufipogon) presents a major problem in commercial cultivation. Red rice produces red coated seeds. After harvest, rice seeds are milled to remove their hull. After milling, domestic rice is white while wild red rice appears discolored. The presence of discolored seeds reduces the value of the rice crop. Since red rice belongs to the same species as cultivated rice (Oryza sativa), their genetic makeup is very similar. This genetic similarity has made herbicidal control of red rice difficult.
Domestic rice tolerant to imidazolinone herbicides have been developed and are currently marketed under the tradename CLEARFIELD®. Imidazolinone herbicides inhibit a plant's acetohydroxyacid synthase (AHAS) enzyme. When cultivating CLEARFIELD® rice, it is possible to control red rice and other weeds by application of imidazolinone herbicides. Unfortunately, imidazolinone herbicide-tolerant red rice and weeds have developed.
Acetyl-Coenzyme A carboxylase (ACCase; EC 6.4.1.2) enzymes synthesize malonyl-CoA as the start of the de novo fatty acid synthesis pathway in plant chloroplasts. ACCase in grass chloroplasts is a multifunctional, nuclear-genome-encoded, very large, single polypeptide, transported into the plastid via an N-terminal transit peptide. The active form in grass chloroplasts is a homomeric protein, likely a homodimer.
ACCase enzymes in grasses are inhibited by three classes of herbicidal active ingredients. The two most prevalent classes are aryloxyphenoxypropanoates (“FOPs”) and cyclohexanediones (“DIMs”). In addition to these two classes, a third class phenylpyrazolines (“DENs”) has been described.
A number of ACCase-inhibitor-tolerance (AIT) mutations have been found in monocot weed species exhibiting tolerance toward one or more DIM or FOP herbicides. Further, an AIT maize has been marketed by BASF. All such mutations are found in the carboxyltransferase domain of the ACCase enzyme, and these appear to be located in a substrate binding pocket, altering access to the catalytic site.
DIMs and FOPs are important herbicides and it would be advantageous if rice could be provided that exhibits tolerance to these classes of herbicide. Currently, these classes of herbicide are of limited value in rice agriculture. In some cases, herbicide-tolerance-inducing mutations create a severe fitness penalty in the tolerant plant. Therefore, there remains a need in the art for an AIT rice that also exhibits no fitness penalty. This need and others are met by the present invention.